Method for producing high quality oxide films on substrates

ABSTRACT

A method for providing an oxide film of a material on the surface of a substrate using a reactive deposition of the material onto the substrate surface in the presence of a solid or liquid layer of an oxidizing gas. The oxidizing gas is provided on the substrate surface in an amount sufficient to dissipate the latent heat of condensation occurring during deposition as well as creating a favorable oxidizing environment for the material.

This invention was made with Government support under contract numberDE-AC02-76CH00016, between the U.S. Department of Energy and AssociatedUniversities, Inc. The Government has certain rights in the invention.

The present invention involves a method for growing oxide layers of amaterial on the surface of a substrate. In particular, the presentinvention involves a method of growing metal oxides on the surface ofcompound semiconductors.

BACKGROUND OF THE INVENTION

It is frequently necessary to form a dielectric layer on the surface ofa semiconductor to function as an insulating layer or a protectivebarrier. A common approach to forming such a dielectric involves growingor depositing an oxide layer of some material on the semiconductorsurface. Typically, the material is a metal or the semiconductingmaterial itself and the resulting dielectric layer is the correspondingoxide of the material.

In one approach known to the prior art, oxide layers can be formedutilizing the material in the semiconductor itself. These layers areoften referred to as "native" oxide layers. In this approach, theoxidation of the semiconductor surface by an oxidizing gas forms theresulting oxide layer. This approach produces favorable results whensilicon is involved due to its propensity to form a stable, high qualityoxide (SiO₂). However, compound semiconductors composed of two or moreof the elements found in Groups IIB, IIIA, IVA, VA and VIA of thePeriodic Table are less favorable candidates for such an approach.Compound semiconductors are characterized as having large electronmobilities as well as a range of band gaps that account for theirdesirable properties as semiconductors. However, compound semiconductorsoften do not oxidize uniformly or form stable, native oxides.

For example, even though a material such as gallium arsenide is anexcellent semiconductor, it is a poor candidate for native oxideformation. The current protocol for inducing native oxide formation onGaAs involves inducing the oxidation of the GaAs in the presence of As₂O₃ and oxygen. Under such an approach, the resulting oxide layercontains a high number of defects.

Defects occur when the resulting oxide layer is non-uniformly formed.Such defects lead to high surface state densities at thesemiconductor/oxide layer interface. These high surface densities trapelectrons when a voltage is applied to the semiconductor. This resultsin concentrations of electrons near the interface which interferes withreproducibility in conductivity. Consequently, undesirable fluctuationsin current are produced when a voltage is applied to the semiconductor.

As a result of the foregoing problems in forming native oxide layers oncompound semiconductors such as GaAs, other approaches for depositingforeign metal oxides on the surfaces of compound semiconductors havebeen attempted.

One conventional technique, known in the art as non-reactive sputtering,involves the bombardment of a target material composed of a selectedoxide material, such as SiO₂. As the target material is bombarded undera vacuum, the oxide material is physically removed from the targetmaterial and is subsequently deposited on the surface of thesemiconductor.

Another technique is a plasma-enhanced chemical vapor deposition processwherein vapor phase reactant gases are subjected to radiation, creatingan ionized plasma. The ionized reactants subsequently interact to formthe desired reaction product which is ultimately deposited on thesubstrate surface.

A shortcoming of both of the aforementioned approaches is that there isa bombardment of the semiconductor surface by either electrons, ions orthe oxide molecules being deposited. This bombardment damages thesubstrate and results in a high number of defects at thesemiconductor/oxide layer interface when attempting to form a metaloxide layer on the surface of compound semiconductors such as GaAs.

Another approach, disclosed in U.S. Pat. No. 4,371,587 to Peters,involves photochemical vapor deposition of the desired oxide material atroom temperature. In particular, the Peters reference involves thephotochemical generation of neutral oxygen atoms by exposing anon-reactive, oxygen-containing gas to collisions with atoms of mercuryvapor that have undergone photochemical excitation. The resultingneutral oxygen atoms are reacted with a compatible vapor phase reactantgas to produce the desired oxide vapor, which subsequently undergoesvapor deposition on the semiconductor surface.

A major shortcoming of the vapor deposition approach previouslymentioned is that as the oxide molecules are deposited as a vapor, theyhave an inherent kinetic energy which is a function of their mass andvelocity. When a particle having a kinetic energy is deposited on asurface, kinetic energy is transferred to the receiving surface in theform of heat energy, known as the latent heat of condensation. If thislatent heat of condensation is not properly dissipated, it can promotewhat is known as an exchange reaction at the interface between the oxidelayer being deposited and the underlying semiconductor surface.

In an exchange reaction, the latent heat of condensation disrupts thebonds occurring between the atoms which make up individual molecules ofthe semiconducting substance. This phenomenon typically occurs at thesurface where the oxide layer is being deposited. Once these bonds havebeen broken, the individual atoms present in the semiconductor surfaceare free to form new bonds. As a result, some of the atoms recombinewith atoms in the oxide layer, forming the undesirable defects discussedpreviously.

As a result, prior art methods which do not adequately address thedissipation of the latent heat of condensation fail to produce a highquality metal oxide layer having a reduced incidence of defects at thedielectric/semiconductor interface.

Furthermore, it is known that inert gases can be retained on the surfaceof a substrate to buffer the latent heat of condensation which occurs asa result of kinetic energy inherent in particles being deposited on thesubstrate surface. However, experimental data has suggested thatreactive gases, especially those containing oxygen, are undesirablebuffers for dissipating the latent heat of condensation because theypromote oxidation of the underlying substrate and result in exchangereactions that impair the quality of the dielectric layer. Indeed, thedifficulties in using a reactive oxidizing gas are readily apparent inthe protocol for creating native oxides of GaAs. Typically, theoxidation of the underlying substrate is difficult to control, despitethe fact that oxygen is present predominantly in a gaseous phase ratherthan being concentrated on the substrate surface as in the presentinvention.

It is therefore an object of the present invention to provide a new andimproved method for depositing an oxide film of a material on asubstrate surface by reactive deposition of the material onto thesurface in the presence of a solid or liquid layer of an oxidizing gas.

It is also an object of the present invention to provide a high qualitymetal oxide layer on the surface of a sensitive semiconductor substance,minimizing the incidence of defects at the oxide-semiconductor interfacecaused by chemical reactions associated with the latent heat ofcondensation during deposition as well as creating a favorable oxidationenvironment for reactive deposition.

SUMMARY OF THE INVENTION

The present invention is a new method for providing an oxide film of amaterial on the surface of a substrate. The method includes providing aliquid or solid layer of an oxidizing gas on the substrate surface in anamount sufficient to promote formation of an oxide of the material onthe substrate. After the layer of oxidizing gas is present on thesubstrate surface, the material is subsequently reactively depositedonto the substrate. Reactive deposition involves depositing a materialonto a receiving matrix wherein the material being deposited undergoes achemical reaction with some substance to formulate a new species at thesite of deposition. In the method of the present invention, thedeposited material reacts with the oxidizing gas present on thesubstrate surface to form the corresponding oxide of the material.

In one embodiment of the method of the present invention, the substrateis preferably a semiconducting substance and the deposited material isan elemental metal. More preferably, the deposited material is eithertitanium, tungsten, aluminum or zirconium and the substrate is acompound semiconductor composed of elements selected from the groupconsisting of periodic Groups IIB, IIIA, IVA, VA and VIA. Mostpreferably, the substrate is gallium arsenide and the deposited materialis aluminum. The oxidizing gas is preferably molecular oxygen. The stepwhich provides for the layer of oxidizing gas on the substrate surfaceis preferably accomplished under a vacuum of ≦10⁻⁹ Torr and atemperature of ≦49° K.

One advantage of the method of the present invention is that it producesan oxide layer that is relatively stable. Another advantage of themethod of the present invention is that the resulting oxide layer is ofa high purity and the substrate/oxide layer interface is free fromundesirable chemical defects. For example, the condensation of theoxidizing gas on the substrate surface does not cause appreciableoxidation of the substrate and a stable oxide of the desired materialforms upon deposition of that material. The oxidizing gas alsosuppresses the substitutional interface which typically occur when thematerial is deposited directly upon the substrate surface.

For a better understanding of the present invention, together with otherand further objects, reference is made to the following description,taken together with the accompanying drawings, the scope of which willbe pointed out in the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows various valence band energy distribution curvescorresponding with the formation of aluminum oxide on a GaAs (100)surface under various conditions.

FIG. 2 shows various energy distribution curves corresponding with thebinding energies associated with the 3d core level electrons of the "Ga"component of GaAs (100), taken at various succesive stages of alumimumoxide formation.

FIG. 3 shows various energy distribution curves corresponding withbinding energies associated with the 3d core level electrons of the "As"component of GaAs (100), taken at various stages of aluminum oxideformation.

FIG. 4 shows various energy distribution curves corresponding with thebinding energies associated with the 2p core level electrons of aluminumthat has been deposited on a GaAs (100) surface under variousconditions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the preferred embodiment of the present invention, a compoundsemiconductor such as gallium arsenide (GaAs) is used as a substrateupon which a layer of a foreign metal oxide, such as aluminum oxide isformed. The process involves condensing a layer of an oxidizing gas,such as molecular oxygen, on the surface of the semiconductor. Forexample, one approach known in the art as cryosorption, involvesmaintaining the substrate under a vacuum at very low temperatures,thereby providing an effective retention of the gas on the substratesurface.

It is known in the art that various compound semiconductors can beformed by combining elements present in Groups IIB, IIIA, IVA, VA andVIA of the Periodic Table. The method of the present invention utilizesgallium arsenide by way of example, however, a variety of othercompounds such as InP and HgCdTe are examples of suitable candidates fora compound semiconductors substrate.

The oxide layer of the method of the present invention is composed ofthe corresponding oxide of the material which is deposited upon thesubstrate surface. Typically, this material will be either a simplemetal, transition metal or an alkali metal. Examples of suitablecandidates would be aluminum, tungsten, zirconium and titanium. Themethod the present invention utilizes aluminum, by way of example, toform its corresponding oxide.

In the preferred embodiment of the method of the present invention, thetemperature of the substrate and the gas being condensed on its surfaceare maintained ≦49° K. in order to retain the gas on the surface of thesubstrate and to minimize undesirable exchange reactions occurringbetween the underlying semiconductor substrate and the oxide layer beingformed upon it. Generally, the lower the temperature of the gas and thesubstrate, the more readily the latent heat of condensation will bedissipated, thereby avoiding undesirable exchange reactions induced bythe disruption of the chemical bonds within individual molecules of thesemiconductor.

An additional source of heat energy, which must also be dissipated inorder to minimize exchange reactions, is produced by the negative heatsof formation being released as the oxide layer is forming. For example,aluminum atoms present in aluminum oxide are in a lower, more stableenergy state than they are when in an uncombined, elemental form. Whenaluminum is combined with oxygen, energy is given off in the form ofheat which must be dissipated in order to avoid exchange reactions.Since the actual chemical combination of the elements which comprise theoxide layer is actually occurring on the semiconductor surface,maintaining the substrate and the gas at ≦49° K. further promotes areduction in the incidence of defect states by effectively dissipatingthese heats of formation.

The preferred embodiment of the method of the present invention alsoutilizes a vacuum of ≦10⁻⁹ Torr to control the rate of condensation ofthe oxygen retained on the substrate surface.

The oxidizing gas performs several functions. On one hand, it functionsas a buffering layer to absorb the latent heat of condensation createdduring the deposition of an elemental metal, such as atomic aluminum,which provides the base material from which the corresponding oxide isformed. Secondly, the oxidizing gas provides oxygen for the formation ofthe oxide being formed.

Contrary to the suggestions of previous experiments, the method of thepresent invention includes providing a condensed film of an oxidizinggas directly on the substrate surface as well as the reactive depositionof a material, thereby resulting in the formation of the desired oxidedirectly on the substrate surface.

Once the oxidizing gas has been provided on the surface of thesubstrate, the base material of which the oxide is formed is deliveredto the substrate surface where it reacts with the oxidizing gas to formthe corresponding oxide. In the preferred embodiment of the presentinvention, atomic aluminum is delivered to a GaAs surface aftermolecular oxygen has been provided on the surface to dissipate thelatent heat of condensation and provide a favorable oxidizingenvironment for producing the aluminum oxide layer.

Unlike the prior art which calls for either a sputtering of a targetmaterial containing the previously formed oxide of choice followed bythe subsequent deposition of the oxide molecules on the substrate, oralternatively, the formation of the oxide vapor by chemical interactionof vapor phase reactants followed by a subsequent vapor phase depositionof the oxide vapor, the method of the present invention involves areactive deposition wherein the oxide is actually formed at the site ofdeposition simultaneously with the delivery of the base material fromwhich the corresponding oxide is formed. More specifically, the materialbeing deposited undergoes a chemical reaction with the oxidizing gas topromote formation of an oxide directly on the substrate surface.

EXAMPLE

In the preferred embodiment of the invention, an electronic grade(100)-oriented GaAs wafer (obtained from Crystal SpecialtiesInternational, Akzo Electronic Products Group, 2853 Janitell Road,Colorado Springs, Colo.) was used as a substrate. The wafer was cut andpolished to within 2°±0.5° off the (100) direction, which was normal tothe wafer surface. Prior to installation in the vacuum system, the waferwas precleaned by etching in a aqueous solution consisting of sulfuricacid (95-98% A.C.S. reagent grade), hydrogen peroxide (30% solution inwater, A.C.S. reagent grade) and distilled water in a ratio of 5:1:1.The etching was done in a room temperature solution for a time rangingfrom 30 seconds to 1 minute. This produced a mirror-like surface on thewafer. The specifics of the etching treatment can be varied to changethe etching speed and surface quality. The wafer had linear dimensionsof 1 cm by 1 cm, but the size could vary from 1 cm² to the 4 inchdiameter or larger disk-shaped wafers normally used in electronic devicefabrication.

The 1 cm² wafer was mounted on a helium cooled copper cold fingercapable of reaching temperatures below 50° K. (-223° C.). The coppercold finger consisted of a solid 3/4 inch diameter cooper rod which wasmodified by drilling a perpendicular hole 1/2 inch in diameter toaccommodate an E-292 ceramic button heater (obtained from Spectra-Mat,Inc., 1240 Highway One, Watsonville, Calif.). The copper rod was furthermodified by machining a threaded stud for installing the rod on aHelitran cryostat (obtained from Air Products and Chemicals, Inc.,Allentown, Pa.). The copper rod was also modified by milling a "flat"perpendicular to the hole drilled for the button heater. This flat wasprovided to facilitate the attachment of the planar GaAs wafer. The 1cm² GaAs (100) wafer was attached to the cold finger using non-magneticstainless steel clips on each corner. The button heater was insertedinto the hole which was drilled in the copper cold finger in order toheat the GaAs wafer from behind. The size and configuration of the GaAswafer processing system was dictated by the peculiar requirements of thespecific vacuum system employed but could, in other cases, vary in sizeand construction within parameters known to practitioners in the field.Additionally, other means known in the art for heating or cooling thesample could be employed.

The cold finger was inserted through a horizontal port of the vacuumchamber and was aligned along the vertical centerline. This orientationwas governed by the geometry of the evaporation sources, the gas sourceand the surface analytical equipment present in the vacuum chamber.However, alternative equipment set-ups could be employed to accommodateother vacuum chamber layouts. The vacuum chamber was an ultra-highvacuum chamber of standard design and consisted of a non-magneticstainless steel vacuum enclosure equipped with a turbo-molecular pump, acryopump and an ion pump. This system maintained a vacuum better than1×10⁻⁹ Torr during the experiment, however, a vacuum ≦1×10⁻⁵ Torr couldbe acceptable.

An oxygen gas source consisting of a cylinder filled with molecularoxygen gas of research grade purity (99.998%), a gas handing manifoldand a leak valve (Model No. 951-5106, obtained from Varian, Inc., PaloAlto, Calif.) was used to supply the oxidizing gas. The leak valve wasdirectly connected to the vacuum chamber. Aluminum was deposited on thewafer using a resistively heated tungsten basket evaporation sourceoperated using procedures well known in the art. A range of commerciallyavailable MBE evaporation sources could also be used to provide aluminumvapor for aluminum oxide growth. During the aluminum evaporation, thesource was operated using power levels that gave an evaporation rate of2 angstroms/minute.

The vacuum-based apparatus described above was used as follows to grow arelatively pure aluminum oxide layer on GaAs (100). The GaAs (100) waferwas cleaned by argon ion bombardment and annealed to restore surfacecomposition and structure. The wafer was subsequently cooled to atemperature ≦49° K. by passing cold helium gas and liquid through theHelitran cryostat. This step typically takes from about 30 to 40 minutesand requiress several liters of liquid helium to cool the cryostat andcold finger to temperatures ranging from about 4.2° K. at the base ofthe cryostat to about 49° K. at the end of the copper cold finger.

The sample was heated during the cooling phase to maintain itstemperature above 250° K. in order to prevent the cryopumping ofimpurities from the residual atmosphere in the vacuum chamber. Thisrequires a gradual increase in the heater power as the copper coldfinger becomes colder. During cooling, the residual atmospheric gases"freeze out" onto the cryostat and colder parts of the copper coldfinger and the vacuum chamber pressure decreased from a pressure ofabout 5×10⁻¹⁰ Torr before cool-down to about 5×10⁻¹¹ Torr. Aftercryopumping, the power was cut to the ceramic button heater and the GaAs(100) wafer rapidly underwent cooling to a temperature of ≦49° K. Therewas little residual gas present in the vacuum chamber atmosphere tocondense on the sample and contaminate it.

After cooling the GaAs wafer to a temperature of ≦49° K., oxygen gas wasadmitted to the vacuum chamber and a multilayer film of molecular oxygenwas deposited on the GaAs substrate. The multilayer molecular oxygenfilm used in this example was deposited by increasing O₂ partialpressure to about 1×10⁻⁷ Torr for 16 minutes. The critical temperaturefor oxygen retention on the surface varies according to the oxygen vaporpressure. In the specific example described herein, oxygen is removedfrom the substrate at approximately 50° K. Consequently, there was arequirement to maintain the substrate at a temperature of ≦49° K. inorder to retain a significant amount of molecular oxygen on thesubstrate surface.

The aluminum oxide layer was grown by depositing atomic aluminum intothe oxygen layer using low-pressure aluminum vapor from the resistivelyheated aluminum source. The aluminum reacts on contact with themolecular oxygen and consumes the molecular gas as a reactant. Theamounts of oxygen and aluminum can be controlled using techniques wellknown to vacuum technologists in order to produce aluminum oxide layersranging in thickness from a few atomic layers to thousands of suchlayers.

Surface spectroscopic data was taken to demonstrate the efficacy of thetechnique described herein for producing a relatively pure aluminumoxide layer on the GaAs (100) surface. Thermal stability of theresulting oxide layer was demonstrated by heating the aluminum oxidecovered GaAs (100) wafer to temperatures of ≧573° K. (300° C.). Noappreciable change in the surface spectroscopic data was observed.Heating at higher temperatures (>650° K.) leads to the destruction ofthe GaAs (100) substrate.

CONCLUSIONS AND SUMMARY OF DATA

In the preferred embodiment of the method of the present invention,valence band and core level photoelectron energy distribution curveswere obtained to study the formation of the interface created betweenthe substrate and the oxide layer. In particular, FIGS. 1-4 have beenprovided to illustrate whether the condensed oxygen retained on thesurface of the GaAs (100) substrate promotes an unacceptable level ofGaAs (100) oxidation, whether the physiosorbed oxygen protects the GaAs(100) from undergoing exchange reactions with the free aluminum beingdelivered to the substrate surface prior to the formation of an aluminumoxide and whether the resulting aluminum oxide overlayer issubstantially clean, uniform and free from other undesirablemetal-oxygen species.

The data illustrated in FIGS. 1-4 shows that the condensation of oxygenon the surface of the GaAs (100) does not result in appreciable levelsof oxidation of the substrate. Additionally, the Figures show that astable metal oxide forms upon the reactive deposition of free aluminum,yielding a high quality oxide film that is free from the exchangereactions which characterize the substrate/oxide layer interface whenfree aluminum is deposited directly on a GaAs substrate without thepresence of an oxygen layer retained on the substrate surface.

The experiments supporting the data present in FIGS. 1-4 were carriedout at the National Synchrotron Light Source of the Brookhaven NationalLaboratory in Upton, N.Y., using beamline U7b (15≦nν≦160 eV).Photoelectron energy analysis was done using a Leybold 100 mmhemispherical analyzer.

In FIG. 1, several valence band energy distribution curves are shown toillustrate the formation of aluminum oxide on a GaAs (100) surface atvarious successive stages. In particular, energy distribution curves(a-d) have been rescaled to comparable heights for visual clarity andshow valence band energies for GaAs (100) under various conditions.

Curve (a) represents the characteristic binding energy for the valenceelectrons of GaAs (100) prior to any exposure to molecular oxygen offree aluminum. The valence band maximum (VBM) was found to be 0.42 eVbelow the Fermi level, indicating the sputter-annealed surface ispinned.

Curve (b) represents the characteristic binding energy for the valenceelectrons of GaAs (100) after exposure to 100 Langmuirs of oxygen at 49°K. Additionally, curve (b) illustrates the presence of physiosorbedmolecular oxygen denoted by the four peaks which have been designated inthe Figure as the O₂ 3δ_(g) (BE=13.6 eV), 3δ_(g) (BE=11.2 eV), 1π.sub.μ(BE=9.8eV), and 1πg(BE=5.2eV) orbitals. These peaks correlate with thebinding energy for the valence electrons of the physiosorbed molecularoxygen. Comparing curve (a) with curve (b), the presence of physiosorbedmolecular oxygen in curve (b) can be observed and is indicative of theO₂ layer present on the substrate surface.

Curve (c) represents the characteristic binding energy for the valenceelectrons of GaAs (100) after 10 angstroms of aluminum have beendeposited. The absence of the O₂ peaks which were present in curve (b)is a result of the free aluminum reacting with the physiosorbedmolecular oxygen to create aluminum oxide, It can be further noted thata new peak is present in curve (c) at ≈6.0 eV and correlates with thepresence of the aluminum oxide layer formed on the substrate surface.This peak represents the binding energy for the valence electrons ofaluminum oxide.

Curve (d) represents a 100 Langmuir oxygen exposure of a 25 angstromaluminum film at room temperature, which is similar to that seen in bulkaluminum oxidation studies.

In summary, FIG. 1 is presented to demonstrate that molecular oxygen wasindeed deposited on the substrate surface and subsequently combined withaluminum to form aluminum oxide.

FIG. 2 has been provided as it contains the most relevant data in termsof illustrating whether the method of the present inventionsubstantially reduces substrate disruption. In particular, FIG. 2illustrates the energy distribution curves for the 3d core levelelectrons of the "Ga" component of the semiconductor substrate, GaAs(100), taken at various successive stages of aluminum oxide formation onthe substrate surface. Curves (a-e) have been rescaled to the sameheight for visual clarity and rigid shifts for curve (e), -0.4 eV, andcurve (d), -0.26 eV, have been introduced to cancel the Fermi levelmovements.

Curve (a) represents the energy distribution curve corresponding withthe binding energies associated with the 3d core level electrons of Gaprior to the exposure of the substrate to O₂ or free aluminum.

Curve (b) represents the energy distribution curve corresponding withthe binding energies associated with the 3d core level electrons of Gaat 49° K. after 100 Langmuirs of O₂ have been deposited on the substratesurface.

Curve (c) represents the energy distribution curve corresponding withthe binding energies associated with the 3d core level electrons of Gaat 49° K. after 2 angstroms of free aluminum have been deposited on thesubstrate surface.

Curve (d) represents the energy distribution curve corresponding withthe binding energies associated with the 3d core level electrons of Gaat 49° K. after 10 angstroms of free aluminum have been deposited on thesubstrate surface.

In a comparison of curves (a)-(d), a small hump-like peak is observablein curves (b) and (c) at 17.4 eV which corresponds to a O₂ 2δ.sub.μpeak. The presence of this peak in curves (b) and (c) indicates thepresence of physiosorbed molecular oxygen. It is noteworthy that thispeak persists after 2 angstroms of free aluminum have been deposited(see curve c) but is not observable after 10 angstroms of aluminum havebeen deposited (see curve d). Such an observation indicates that thefirst 2 angstroms of aluminum do not consume all of the molecular oxygenand that oxidation continues until all of the molecular oxygen isexhausted.

Curve (e) represents the energy distribution curve corresponding withthe binding energies associated with the 3d core level electrons of Gafor a 10 angstrom aluminum deposition of GaAs (100) at room temperaturewithout any molecular oxygen retained as a film on the substratesurface. A substitutional interface or exchange reaction involving theformation of an undesirable Ga-Al species is indicated on curve (e) by ashifted Ga 3d component having a ΔBE≈-1 eV.

Comparing curves (e) to curves (b)-(d) indicates that the deposition offree aluminum on the substrate surface by the method of the presentinvention does not cause the Ga-Al exchange reaction, thereby yielding ahigh quality oxide layer free from defect states.

FIG. 3 illustrates various energy distribution curves corresponding withthe binding energies associated with the 3d core level electrons of the"As" component of the semiconductor substrate, GaAs (100), taken atvarious successive stages of aluminum oxide formation.

In particular, curve (a) represents the characteristic energydistribution curve corresponding with the binding energies associatedwith the 3d core level electrons of As when present in the form of GaAs(100), prior to any exposure to oxygen or free aluminum.

Curve (b) corresponds to data taken after the substrate surface has beenexposed to 100 Langmuirs of O₂ at 49° K. The similarity between curves(a) and (b) indicates that there has been no disruption of the substratesurface with respect to As upon exposure to O₂. If the As component ofthe GaAs (100) had reacted with O₂, an additional peak would have beenobserved.

Curve (c) represents the characteristic binding energies associated withthe 3d core electrons of As present in GaAs (100) at 49° K. after havingreceived condensed O₂ on its surface and after having received thedeposition of 2 angstroms of free aluminum. A discernible peak appearsat ΔBE=3.1 eV which corresponds to an As oxide intermediate.

Upon examination of curve (d), which correlates to data taken after 10angstroms of aluminum were deposited, it is apparent that the additionalAs oxide peak appearing in curve (c) has disappeared upon the depositionof additional aluminum, indicating that the formation of As oxide isintermediary.

Curve (e) represents the energy distribution curve corresponding withthe binding energies associated with the 3d core level electrons of Aswhen present in the form of GaAs (100), after 10 angstroms of aluminumwas directly deposited on the substrate surface at room temperature.Surprisingly, there is no disruption of the substrate under normal roomtemperature conditions, as indicated by the similarity between curves(e) and (a). Thus, from the data gathered from the standpoint of As 3dcore level electron binding energies, it would appear that there is nosubstrate disruption under any conditions. Taken alone, FIG. 3 mightindicate there is no advantage in the present invention for providing abetter method for depositing an oxide layer on a semiconductor surfaceunder low temperatures. However, the data present in FIG. 2 is basedonly on the energy distribution curves for the 3d core level electronsof As and is not conclusive as to the effectiveness of the presentinvention, especially in light of the fact that it is the "Ga" componentof GaAs which tends to undergo the problematic exchange reactionspreviously discussed.

Finally, FIG. 4 has been provided to demonstrate that the resultingoxide layer is uniformly composed of aluminum oxide, free from anymetallic aluminum. In particular, FIG. 4 illustrates the characteristicbinding energies for the 2p core level electrons of aluminum that hasbeen deposited on a GaAs (100) surface in the presence of a solid layerof molecular oxygen in order to induce aluminum oxide growth.

Curve (a) of FIG. 4 represents the energy distribution curvecorresponding with the binding energy of the 2p core level electrons ofaluminum after the deposition of two angstroms of free aluminum on aGaAs (100) surface which has received 100 Langmuirs of O₂ at atemperature of 49° K. Curve (a) contains a peak corresponding to an Al-Ocomponent having a chemical shift of ≈1.5 eV that suggests that the Al-Oenvironment is aluminum rich compared to Al₂ O₃ or the surface Al-Ophase seen during the early stage of polycrystalline aluminum oxidation.

Curve (b) represents the energy distribution curve corresponding withthe binding energies associated with the 2p core level electrons after10 angstroms of aluminum have been deposited. A shoulder appears on theAl-O peak indicating the presence of metallic aluminum. The appearanceof this shoulder at BE=72.6 eV coincides with the disappearance of theO₂ 2δ.sub.μ feature seen in curve (d) of FIG. 2, indicating that all ofthe molecular oxygen has been consumed. Additionally, it can be observedthat the deposition of 10 angstroms of aluminum also removes oxygen fromthe As₂ O₃ component appearing in curve (d) of FIG. 3.

Curve (c) represents the energy distribution curve corresponding withthe binding energies associated with the 2p core level electrons ofaluminum for aluminum deposited on a GaAs (100) surface at roomtemperature. Only a peak corresponding with metallic aluminum and abroadening of that peak due to the presence of an unresolved Ga-Alcomponent are observed. It should be noted that the metallic aluminum 2pposition (BE=72.9 eV), after the deposition of 10 angstroms of aluminum,is not exactly equal to the bulk aluminum 2p value. However, after 25angstroms of aluminum have been deposited, the aluminum 2p peak positionmoves to 72.4 eV. This is the same peak position as seen when a thickfilm of aluminum is deposited on a substrate surface (see, for example,curve d).

Curve (d) represents the energy distribution curve corresponding withthe binding energies associated with the 2p core level electrons ofaluminum after aluminum has been deposited as a thick film (>30angstroms) on a tantalum substrate.

In comparing curves (a-d) of FIG. 4, it is apparent that the depositedaluminum reacts completely with molecular oxygen to form an aluminumoxide before all the molecular oxygen is exhausted and forms a completeoxide layer with no trace of metallic aluminum.

In conclusion, the data shows that the condensation of oxygen does notcause oxidation of the underlying substrate, that a stable metal oxideforms upon metal deposition and that the condensed oxygen suppresses theGa-Al substitutional interface reaction which occurs when aluminum isdeposited directly on a GaAs surface.

While there have been described what are presently believed to be thepreferred embodiments of the invention, those skilled in the art willrealize that changes and modifications may be made thereto withoutdeparting from the spirit of the invention, and it is intended to claimall such changes and modifications as fall within the true scope of theinvention.

We claim:
 1. a method for providing an oxide film of a material on asubstrate surface comprising:providing a solid or liquid layer of asubstance which is an oxidizing gas at room temperature on saidsubstrate surface in an amount sufficient to promote formation of anoxide of said material upon said substrate surface at a pressure of lessthan or equal to 10⁻⁵ Torr and temperatures less than or equal to 49°K.; and reactively depositing said material onto said substrate surfacein the presence of said layer of said oxidizing gas.
 2. The method ofclaim 1, wherein said substrate surface comprises a semiconductingsubstance.
 3. The method of claim 1, wherein said material is anelemental metal.
 4. The method of claim 2, wherein said semiconductingsubstance is a compound semiconductor.
 5. The method of claim 4, whereinsaid compound semiconductor is composed of elements selected from thegroup consisting of periodic Group IIB elements, periodic Group IIIAelements, periodic Group IVA elements, periodic Group VA elements andperiodic Group VIA elements.
 6. The method of claim 4, wherein saidcompound semiconductor is composed of elements selected from the groupconsisting of periodic Group IIIA elements, periodic Group IVA elementsand periodic Group VA elements.
 7. The method of claim 4, wherein saidcompound semiconductor is composed of gallium arsenide.
 8. The method ofclaim 3, wherein said elemental metal is selected from the groupconsisting of titanium, tungsten, aluminum and zirconium.
 9. The methodof claim 3, wherein said elemental metal is aluminum.
 10. The method ofclaim 1, wherein said oxidizing gas is molecular oxygen.